The invention relates to a method of producing a mixed substrate, i.e. a substrate which comprises portions of layers of insulating materials buried within a substrate of semiconductor material. Such a substrate may be used to produce both microelectronic components with a buried insulating layer and components without a buried insulating layer.
Interest in mixed substrates is currently increasing. Alternating bulk zones and insulating zones within the same substrate allows different components to be produced on one and the same substrate wafer such as, for example:                on bulk zones: components having an electrical connection between the front and back faces of the substrate, for example components termed “vertical” components;        and on “SeOI (semiconductor on insulator) or “SOI” (silicon on insulator) type zones, components which are completely isolated from each other and isolated from their substrate, for example “MOS” type components or “MEMS” or “MOEMS” type systems.The acronym “MOS” means “metal oxide semiconductor”. The acronyms “MEMS” and “MOEMS” mean “micro electro mechanical systems and “micro-opto-electro-mechanical systems, respectively”.        
A number of techniques exist for fabricating mixed compounds. In particular, the technique employing the method known by the name of SIMOX may be mentioned. That method consists in implanting oxygen beneath the surface of a silicon substrate, then annealing that substrate at high temperature to convert the oxygen-implanted region into silicon dioxide (SiO2). By way of example, an application of that SIMOX method to the production of mixed substrates is described in U.S. Pat. No. 6,846,727. That method consists in using a mask to implant oxygen ions into the substrate in a localized manner and then to create therein isolated layers of silicon oxide (SiO2) using a suitable heat treatment. However, the surface quality of the mixed substrate obtained is mediocre and non-uniform, due to the increased volume of the SiO2 within the silicon substrate. Further, a large number of defects are also observed in the transition regions between the bulk zones and the SOI type zones.
Furthermore, according to U.S. Pat. No. 6,955,971, a method of fabricating a mixed substrate is known that uses molecular bonding to bring a first substrate comprising insulating zones into contact with a second substrate from which the transferred layer is to be derived. In order for that type of bonding to be of high quality, the surface quality of the substrates to be bonded has to be perfect, i.e., without the slightest defect. Ideally, the bonding interface must not induce any disturbance in future components which will use its junctions. Thus, the surfaces must be carefully prepared prior to bonding using chemical, thermal, plasma, and/or chemical-mechanical polishing treatments, for example.
Polishing is a critical step. However, because the support substrate carries mixed zones, i.e., bulk zones of silicon and insulating zones of silicon oxide, materials which have different natures are not polished at the same rate. Thus, it is difficult for the surface roughness of those two materials to be good on the same substrate.
In practice, after polishing, a difference in the level between the two types of materials, or “dishing”, is observed on the surface of the support substrate. This dishing, when it is negative, may be by as much as tens of nanometers, depending on the dimensions of the silicon oxide zones and/or the silicon zones. The negative dishing is principally observed at the silicon zone, as that material is polished more rapidly than the oxide. Because of those surface irregularities, molecular bonding is imperfect and the good quality bonding interface which would be necessary for the subsequent production of good quality components cannot be obtained. Thus, yields are very poor.
To improve the surface planarity of a mixed substrate surface with a view to bonding it and to overcome dishing problems, U.S. Pat. No. 5,691,231 proposes depositing a layer of polycrystalline silicon and polishing it on the substrate after forming and polishing the oxide. However, the electrical properties of a bonding interface between a layer of polycrystalline silicon and a layer of monocrystalline silicon are not sufficient for future components to be fabricated. Further, the presence of a layer of polycrystalline silicon in a substrate requires that the usual component fabrication technologies be further developed or adapted. They were in fact originally developed for substrates comprising a layer of monocrystalline silicon transferred onto an insulating layer. However, a particular mode of preparation of polycrystalline silicon is necessary for direct bonding, both as regards its deposition and its planarization, thereby increasing the number of steps of the fabrication process and also its cost. Finally, inhomogeneities accumulate in the various polishing steps carried out, namely in the first polish of the mixed zones and the second polish of polycrystalline silicon, which has the effect of causing wide variations in the total thickness of the transferred silicon (monocrystalline silicon and polycrystalline silicon).
Finally, International patent application WO-2004/059711 describes a method which consists in bonding two substrates one against the other by molecular bonding, one of those substrates having zones of insulating layers. In order to obtain a good quality bonding interface, impurity traps are created in one of those substrates. Those traps are themselves insulating zones and are disposed so that a short distance exists between two successive insulating zones. Those insulating zones will trap and absorb the impurities which essentially appear during heat treatment applied to strengthen the bonding interface. However, that type of mixed substrate suffers from the disadvantage that the distance between two insulating layer zones must be extremely small so that those zones can act as traps for impurities. Such a method, however, cannot produce mixed substrates with relatively large bulk zones.
In general, it is known that oxide layers present in SOI type substrates can absorb impurities. They are, for example, precipitates of native oxide and/or molecules of gas enclosed when the two substrates are brought into contact, or are already present in those substrates. Those oxide layers can thus indirectly have a good bonding interface without bonding defects, particularly with direct bonding. However, a method of producing mixed substrates by molecular bonding with good quality interfaces despite the absence of insulating layers on certain portions of the support substrate currently does not exist.
Accordingly, the present invention proposes to overcome these difficulties and fill the shortcomings mentioned above by providing improved methods for making such mixed substrates.